opentelemetry: add opentelemetry source code attributes to spans (#1411)

The opentelemetry specification calls for a number of attributes to correlate
traces to their location in the code. They are documented here [1]. This commit
adds support for the following fields based on the tracing span metadata (all
relative to span creation):

- `code.namespace`: Crate & module path (`my_crate::my_mod`)
- `code.filepath`: Relative path to the source code file (`src/my_mod.rs`)
- `code.lineno`: Line number in the file indicated by `code.filepath` (`72`)

As written this will annotate all spans with these attributes. If we want to be
a bit more conservative, I could add an instance variable to the Subscriber that
allows users to opt-in or opt-out of this functionality.

[1]: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/semantic_conventions/span-general.md#source-code-attributes

Co-authored-by: Lily Mara <lilymara@onesignal.com>

tracing-opentelemetry/src/layer.rs

@@ -421,6 +421,22 @@ where
             builder.trace_id = Some(self.tracer.new_trace_id());
         }
 
+        let builder_attrs = builder.attributes.get_or_insert(Vec::new());
+
+        let meta = attrs.metadata();
+
+        if let Some(filename) = meta.file() {
+            builder_attrs.push(KeyValue::new("code.filepath", filename));
+        }
+
+        if let Some(module) = meta.module_path() {
+            builder_attrs.push(KeyValue::new("code.namespace", module));
+        }
+
+        if let Some(line) = meta.line() {
+            builder_attrs.push(KeyValue::new("code.lineno", line as i64));
+        }
+
         attrs.record(&mut SpanAttributeVisitor(&mut builder));
         extensions.insert(builder);
     }